In buildings for households and offices, installation of a radio communication base station apparatus (a femto base station or a home node-B, also referred to as “HNB”) that covers a small cell or a femtocell having a cell radius of several tens of meters is considered. FIG. 1 shows the configuration of a radio system including an HNB. A mobile terminal located in the buildings in which an HNB is installed transmits data to and receives data from the HNB via a radio channel. The HNB uses a fixed-line network such as an optical fiber as a backbone to connect with a core network via a concentrator (GW).
When HNBs become prevalent, it is expected that a femtocell and a macrocell share the same frequency band in urban areas and are operated in the hierarchical cell arrangement shown in FIG. 1. In addition, it is also expected that access to an HNB is limited only to a registered user (a closed subscriber group, CSG). Such an operation, however, raises a problem of increased uplink radio channel interference between a macrocell and a femtocell. That is to say, there is a possibility that a tradeoff occurs in which, when throughput increases in one cell, the other cell suffers increased radio interference and decreased throughput.
One example of uplink radio channel interference between a macrocell and femtocells is uplink radio channel interference from user equipment connected to the macrocell (hereinafter referred to as “MUE”) to the HNB. In particular, MUE radio transmission power increases when the distance between the macro base station (hereinafter referred to as “MNB”) and the MUE becomes greater. Therefore, when a femtocell is located at a macrocell edge, it is necessary to take measures against interference from the MUE that does not have an access right to the HNB. As specific measures, Non-Patent Literature 1 describes increasing the target control value of total HNB reception power depending on interference level at an HNB and adjusting receiver gain at the HNB.
A second example of uplink radio channel interference between a macrocell and a femtocell is uplink radio channel interference from user equipment connected to a femtocell (hereinafter referred to as “HUE”) to an MNB. In particular, when the distance between an HUE and an HNB is long and the distance between the HUE and an MNB is short, the amount of interference from the HUE to the MNB increases. Therefore, when a femtocell is located at the center of a macrocell, it is necessary to suppress interference from an HUE to an MNB. As specific measures, Non-Patent Literature 1 and Patent Literature 1 describe limiting the maximum HUE transmission power.
Now, a method of limiting the maximum HUE transmission power will be described in detail. FIG. 2 is a sequence diagram showing the steps of restricting the maximum HUE transmission power described in Non-Patent Literature 1. FIG. 2 picks up and shows only parts that relate to uplink interference control. The 3rd generation partnership project (3GPP) release. 6 (HSUPA) presumes an uplink access method.
An HNB is provided with a function of measuring a reception level of a macrocell signal (a downlink common pilot channel (CPICH), for example) (measurement section 24). Measurement section 24 measures a macrocell signal reception level when an HNB is started, for example (ST 11). More specifically, as described in Non-Patent Literature 2, the RSCP of a macrocell CPICH is measured and information about P-CPICH transmission power is obtained. Measurement section 24 reports the measurement result to control section 23 (ST 12). Control section 23 uses the reported measurement result to determine the maximum HUE transmission power (ST 13). Control section 23 lowers the set value of the maximum HUE transmission power when the macrocell signal reception level becomes greater.
When the HUE starts connecting with the HNB, radio resource control (RRC) connection is established (ST 14). In this case, control section 23 of the HNB reports a setting of the maximum HUE transmission power to HUE 21. HUE 21 transmits an uplink radio signal within the range of the reported maximum HUE transmission power.
HUE 21 measures the macrocell signal reception level based on the command from the HNB (ST 15), and reports the measured value to the HNB (ST 16). Control section 23 of the HNB updates the set value of the maximum transmission power of HUE 21 based on the value reported from the HUE 21, and reports the set value to HUE 21 (ST 16). HUE 21 transmits an uplink radio signal within the range of the updated and reported maximum transmission power.
FIGS. 3(a) and 3(b) show changes of HNB reception power and HUE transmission power, respectively, when the above-described interference control is used. In FIG. 3, the horizontal axis of the graph indicates path loss between an HNB and an HUE. In FIG. 3(a), the vertical axis of the graph indicates reception power, and, in FIG. 3(b), the vertical axis of the graph indicates transmission power.
As is clear in FIG. 3, the target control value of total HNB reception power (RoT target) is constant regardless of the spatial path loss between an HNB and an HUE (hereinafter referred to as path loss) or the macrocell signal reception level. On the other hand, the maximum value of total HUE transmission power is adjusted depending on the macrocell signal reception level at the HNB.
In FIG. 3, bold solid lines indicate the power value. FIG. 3 shows that total HUE transmission power reaches the maximum value and HNB reception power is lowered when an HUE moves away from an HNB and path loss between the HNB and the HUE becomes greater.
Here, the ratio of the transmission power of HSUPA channel (E-DCH) to the transmission power of dedicated physical control channel (DPCCH) constituting W-CDMA channel (DCH) is defined. It is defined that the power ratio increases (transmission power increases) when the E-DCH transmission rate increases so that required power increases as the E-DCH transmission rate increases.
Because the transmission power of DCH is controlled so that the reception quality at the base station is set at the desired value, the transmission power of E-DCH increases in proportion to DCH when path loss becomes greater. Here, when HUE transmission power reaches the maximum value, the required power that is suitable for the E-DCH transmission rate cannot be secured. In this case, the HSUPA scheduler of the HNB lowers the transmission rate to allocate to the HUE based on the transmission power headroom reported by the HUE (UE power headroom), for example.